There are many applications where three-phase current sensors are used in the industrial field, i.e. for controlling or monitoring three-phase motors. Typically, in three-phase systems, a current sensor comprising a magnetic circuit in a soft often laminated magnetic material with a gap in which a magnetic field sensor is positioned, is placed around each of the three conductors. A very widespread magnetic field sensor is a Hall effect sensor integrated into an ASIC with terminals for its connection to a printed circuit for processing the signals. The electric current may be measured and the electric power may be inferred therefrom by positioning sensors around two of the three phases under the assumption that the sum of the currents is zero. Nevertheless, if the intention is also to detect possible leakage currents, notably for safety reasons, it is necessary to measure the electric current in the three phases and sensors will be positioned around each of the phases.
Measurement of a strong current poses problems in existing sensors and requires special, cumbersome and costly measurements. Strong currents generate high magnetic fields which may saturate the magnetic circuit of the sensor, causing a non-linear response and consequently errors of measurement. This saturation problem is worsened when the conductors of the three phases are brought closer to each other, since the magnetic field of the adjacent conductors is added to the field generated by the conductor around which the relevant sensor is positioned.
In order to avoid saturation of the magnetic circuit, the section of the magnetic circuit may be increased. In many applications (for example in the automotive field), it is however sought to reduce the bulkiness and the weight of the measuring apparatuses. One of the future applications of current sensors is precisely in the field of controlling electrical power systems for electric motor vehicles where the currents may be very strong because of the relatively low voltage and of the high powers required for propelling the vehicle. By increasing the section of the magnetic circuits, the cost of the sensor is also increased.
Another solution which has been considered is to place at least one of the magnetic field sensors in a notch formed in one of the conductors bearing the three phases in order to reduce the number of magnetic circuits and to limit the saturation problems of these circuits. However, making a notch in one of the conductors and mounting a magnetic field sensor in this notch is an expensive solution. Furthermore, the problem of the influence of the magnetic fields generated by the neighboring strong current conductors is still not solved, which limits the possibilities for reducing the bulkiness of the measuring device.